unit testing - Why Create Mock Objects?

Question

During a recent interview I was asked why one would want to create mock objects. My answer went something like, "Take a database--if you're writing test code, you may not want that test hooked up live to the production database where actual operations will be performed."

Judging by response, my answer clearly was not what the interviewer was looking for. What's a better answer?

Answer 1

I'd summarize like this:

• Isolation - You can test only a method, independently on what it calls. Your test becomes a real unit test (most important IMHO)
• Decrease test development time - it is usually faster to use a mock then to create a whole class just for help you test
• It lets you test even when you don't have implemented all dependencies - You don't even need to create, for instance, your repository class, and you'll be able to test a class that would use this repository
• Keeps you away from external resources - helps in the sense you don't need to access databases, or to call web services, or to read files, or to send emails, or to charge a credit card, and so on...

In an interview, I'd recommend including that mocking is even better when developers use dependency injection, once it allows you to have more control, and build tests more easily.